Typically, a torque sensor is applied to an actuator that controls a rotary drive system. The torque sensor is attached to a rotating body that is supported by a bearing and includes a strain part to be strained due to a torsion moment. By measuring the distortion of the strain part, torque of the rotating body is detected. In order to achieve a high-accuracy control, it is required to accurately measure the torque. Various techniques therefor have been developed.
Japanese Unexamined Patent Application Publication No. 2011-209099 discloses a torque sensor that has a structure in which a first rotating body (an inner ring) and a second rotating body (an outer ring) formed of a pair of concentric annular bodies having diameters different from each other are divided from each other, the torque sensor transmitting torque from the inner ring to the outer ring through strain bodies. The strain bodies are integrally formed with the inner ring and are shaft portions (beams) protruding from the outer peripheral part of the inner ring to the inner peripheral part of the outer ring.
The strain bodies are relatively movable relative to the outer ring in the axial direction, the radial direction, and the rotational direction and include engaging parts capable of being engaged in the rotational direction. In the torque sensor, the strain body has a predetermined amount of freedom in the aforementioned three directions, whereby it becomes possible to reduce influences of vibrations occurring in the rotating body and to detect torque with high accuracy.
Japanese Unexamined Patent Application Publication No. 2013-061305 discloses a torque sensor that measures torque transmitted from a primary-side fastening member (an input side) to a secondary-side fastening member (an output side). This torque sensor includes a first structure body that is coupled to the primary-side fastening member, a second structure body that is coupled to the secondary-side fastening member, a strain part that couples the first structure body to the second structure body, and a strain sensor capable of detecting an amount of deformation of the strain part. The first structure body is coupled to the primary-side fastening member by bolted connection. The first structure body and the primary-side fastening member are fitted to each other while having clearance smaller than that of the bolted connection in a torque acting direction (circumferential direction) which is a direction in which the torque acts, by combining a protruding part and a groove part that accommodates the protruding part.
Japanese Unexamined Patent Application Publication No. 2007-040774 discloses a torque sensor in which a load member that receives loads and a torque member that receives torque are constituted independently from each other. The second moment of area of the torque member is made maximum in the direction of rotation (minimum in the direction along the axial center) and the second moment of area of the load member is made maximum in the direction along the axial center (minimum in the direction of rotation), whereby it is possible to detect torque with high accuracy while preventing loads received by the load member from affecting the torque member.
In Japanese Unexamined Patent Application Publication No. 2011-209099, in the torque sensor embedded in the actuator, four engaging parts of four respective strain bodies are separated from engaging recessed portions of the outer ring with predetermined gaps therebetween. According to this structure, transmission of forces and torque other than the torque around the axis to be detected is interrupted and the detection accuracy is enhanced. However, there are limitations in the working accuracy of the engaging recessed portions of the outer ring, the working accuracy of the widths of the engaging parts of the strain bodies, and the working accuracy of the gaps between the engaging recessed portions and the engaging parts that are engaged with each other.
When the widths of the gaps are not equal to one another, four engaging parts do not contact the respective engaging recessed portions equally to transmit the torque and only one engaging part may contact the engaging recessed portion or three engaging parts may contact the respective engaging recessed portions depending on the magnitude of the torque. In this case, the states of deformation in all the four strain bodies are different from one another, which causes bridge imbalance of resistances of detection elements that detect torque. Therefore, it becomes extremely difficult to increase the accuracy for detecting the torque.
Further, the strain bodies are supported through a rotation-transmitting member and a speed reducer on a casing by first and second bearings and the outer ring is supported through another bearing on the casing. As described above, the strain bodies and the outer ring are supported on the casing through bearings different from each other, and it is possible that axial cores of the bearings may be deviated from each other. When the rotating axis of the input side (the speed reducer and the inner ring) of the torque sensor and the rotating axis of the output side (the outer ring and the output axis) thereof are deviated from each other, it is difficult to make the widths of the respective gaps between the engaging parts and the engaging recessed portions equal to one another, and it is possible that an uneven contact, in which only some of the four engaging parts contact the respective engaging recessed portions, may occur. Therefore, an even rotational torque is not given to the strain bodies (an even strain does not occur in the strain bodies), which causes degradation in the accuracy for detecting the torque.